1. Field of the Invention
The present invention relates to a piezoelectric filter including a piezoelectric resonator having a substrate, such as a silicon wafer, and a piezoelectric thin-film disposed on the substrate, and also relates to a communication device and a method for manufacturing the piezoelectric filter.
2. Description of the Related Art
Piezoelectric resonators having a substrate, such as a silicon wafer, and a piezoelectric thin-film disposed on the substrate are disclosed in Japanese Unexamined Patent Application Publication No. 58-121815 and U.S. Pat. No. 5,910,756.
A piezoelectric resonator disclosed in Japanese Unexamined Patent Application Publication No. 58-121815 has a substrate having a recessed portion, a piezoelectric thin-film having a pair of electrodes and disposed on a surface of the substrate, and a frequency-adjusting thin-film disposed in the recessed portion located on the back of the surface of the substrate and located at an area corresponding to the piezoelectric thin-film, in order to control the resonant frequency.
In U.S. Pat. No. 5,910,756, a filter including a plurality of piezoelectric resonators connected to one another in a ladder configuration is disclosed, wherein the piezoelectric resonators each have two SiO2 thin-films in order to control the resonant frequency.
In the configuration disclosed in Japanese Unexamined Patent Application Publication No. 58-121815, when the frequency is adjusted by trimming the frequency-adjusting thin-film in the recessed portion, the frequency-adjusting thin-film must be selectively formed on a desired area corresponding to the piezoelectric resonator via a deposition process. Since space is available between the substrate and a mask for forming the frequency-adjusting thin-film, a component for the frequency-adjusting thin-film is deposited on an area which must be isolated. Therefore, the selectivity of the film formation must be improved but it is difficult to do so which presents a significant problem. Since the frequency-adjusting thin-film is formed for each piezoelectric resonator one after another by deposition, the through-put is small. Therefore, there is a problem in that the productivity cannot be readily increased. Furthermore, when the frequency-adjusting thin-film includes metal, the upper and lower electrodes are electrically coupled with the metal frequency-adjusting thin-film. Therefore, there is a risk that the characteristics of the resonator are deteriorated.
On the other hand, in the configuration disclosed in U.S. Pat. No. 5,910,756, since it is difficult to widely control the resonant frequency with respect to the thickness of the SiO2 layers, the second SiO2 layer must have a certain thickness in order to obtain a desired difference between the frequency of resonators connected in parallel and that of resonators connected in series when a ladder filter is prepared. As a result, the resonators connected in parallel have a ratio of the frequency-adjusting thin-film thickness to the two SiO2 layer thickness which is significantly different from that of the resonators connected in series. The resonators connected in parallel subsequently have the temperature characteristics of a resonant frequency which is significantly different from those of the resonators connected in series. Therefore, there is a problem in that the filter characteristics are significantly changed.